The most common types of residential or commercial HVAC air filters employ a fibrous filter media (made from polyester fibers, glass fibers or microfibers, etc.) placed substantially perpendicular to the airflow through which air may pass (e.g., an air conditioner filter, a HEPA filter, etc.) such that particles are removed from the air mechanically (coming into contact with one or more fibers and either adhering to or being blocked by the fibers); some of these filters are also electrostatically charged (either passively during use, or actively during manufacture) to increase the chances of particles coming into contact and staying adhered to the fibers.
Another form of air filter is known as an electronic air cleaner (EAC). A conventional EAC includes one or more corona electrodes and one or more smooth metal collecting electrode plates that are substantially parallel to the airflow. The corona electrodes produce a corona discharge that ionizes air molecules in an airflow received into the filter. The ionized air molecules impart a net charge to nearby particles (e.g., dust, dirt, contaminants etc.) in the airflow. The charged particles are subsequently electrostatically attracted to one of the collecting electrode plates and thereby removed from the airflow as the air moves past the collecting electrode plates. After a sufficient amount of air passes through the filter, the collecting electrodes can accumulate a layer of particles and dust and eventually need to be cleaned. Cleaning intervals may vary from, for example, thirty minutes to several days. Further, since the particles are on an outer surface of the collecting electrodes, they may become re-entrained in the airflow since a force of the airflow may exceed the electric force attracting the charged particles to the collecting electrodes, especially if many particles agglomerate through attraction to each other, thereby reducing the net attraction to the collector plate. Such agglomeration and re-entrainment may require use of a media afterfilter placed downstream and substantially perpendicular to the airflow, thereby increasing airflow resistance. Another limitation of conventional EACs is that corona wires can become contaminated by oxidation or other deposits during operation, thereby lowering their effectiveness and necessitating frequent cleaning. Moreover, the corona discharge can produce a significant amount of contaminants such as, for example, ozone, which may necessitate an implementation of activated carbon filters placed substantially perpendicular to the airflow that can increase airflow resistance.
While fibrous media filters do not produce ozone, they typically have to be cleaned and/or replaced regularly due to an accumulation of particles. Furthermore, fibrous media filters are placed substantially perpendicular to the airflow, increasing airflow resistance and causing a significant static pressure differential across the filter, which increases as more particles accumulate or collect in the filter. Pressure drop across various components of an HVAC system is a constant concern for designers and operators of mechanical air systems, since it either slows the airflow or increases the amount of energy required to move the air through the system. Accordingly, there exists a need for an air filter capable of relatively long intervals between cleaning and/or replacement and a relatively low pressure drop across the filter after installation in an HVAC system.